Plasma calcium control in the rat fetus. II. Influence of fetal hormones.

Total and ionized plasma calcium concentrations are higher in the fetus than in the mother at the end of gestation. Using thyroparathyroidectomized (TPTX), nephrectomized (NX), or decapitated fetuses from normal or TPTX pregnant rats, we have investigated the influence of some fetal hormones on fetal plasma calcium level. Surgical ablations were performed at 19.5 days of gestation, and the animals were sacrificed 2 days later. TPTX fetuses from normal mothers showed a decrease in fetal plasma calcium (approximately 1 mg/dl), but decapitation, which removes brain, hypophysis, and the thyroparathyroid complex, had no effect. By contrast, in TPTX mothers, decapitated fetuses exhibited a sharp decrease in plasma calcium (2.37 mg/dl), and a rise in plasma phosphate (1.39 mg/dl), but surprisingly, plasma calcium was unchanged in TPTX fetuses. NX fetuses both in normal and TPTX mothers showed unaltered plasma calcium and phosphate levels. Some of these findings are as yet unexplained and suggest the contribution of other unknown fetal hormones in the control of fetal plasma calcium.